In a vehicle, particularly a trailer, the axles are attached to the body of the vehicle through a suspension system. In many trailers, the axles are paired in a tandem axle pair, with the forward and rear axles of the pair attached to a suspension of a type generally known as a trailing arm suspension. Such a suspension system provides two beams for each axle, with one or more axles being provided for the front and the rear of the vehicle. The forward and rear axle beam pairs are attached to pendant brackets from a steel frame superstructure of the vehicle. This connection between the beams and the bracket is made through a bushing, an improvement of which is the subject of the present invention. Each of the beam pairs is likewise attached to the respective axle at the trailing ends of the beams by a pair of welds near the ends of the axle. These welds may be generally characterized as longitudinal welds of the side surfaces of the axle to a saddle piece which is attached to the beam. In some of these suspensions, at least one resilient pneumatic chamber is provided between the beam and the vehicle frame to take up some of the articulation forces and load of the vehicle. These suspensions are generally referred to as "air-ride" suspensions.
As the vehicle moves, the axle is subjected to a variety of forces, but these forces may be generally resolved into three distinct component forces. First, there is a component which acts from side to side on the vehicle, that is, in an axial direction with reference to the axle. The second force is an "up-down", or vertical, radial deflection or oscillation of the axle. The third force is a "fore-aft", or horizontal, radial deflection or oscillation of the axle. Each of these forces acts upon the welds which secure the axle in a non-rotative fashion to the beam. Various means have been devised to protect the weld from the component forces, but premature failure of welds is still considered a problem. It is believed that protection of the weld from the "fore-aft" oscillations will go a long way in prolonging weld life.
Forces acting on the axle may be transmitted through the beam pair to the bracket and ultimately to the frame if not attenuated or damped. In mounting the axle to the frame in a resilient manner to absorb oscillations received by the axle during motion, it is known to use a resilient bushing to isolate metal-to-metal connections at the beam-bracket juncture which would transmit the oscillations. In fact, some bushings of the prior art have been sophisticated enough to provide a radially varying stiffness by providing longitudinal cavities in a resilient member in the bushing. However, it is also known that a resilient bushing by itself does not always resolve the problem, since the pivotal motion permitted by such a bushing may establish a resonance which may be just as damaging as the initial oscillation. Although the prior art teaches radial variance in stiffness of the bushing, it does not teach the spring rate variance in opposing directions of a diameter of the bushing.